Most hardy geraniums do not bloom on into the fall however last year sanguineum var.striatum surprised us by still having a few blooms right up to the first snow. I am starting Geranium wlassovianum (Siberian cranesbill) from seed just because it is supposed to be a late summer bloomer. Geranium wallichianum 'Buxtons Blue' is also supposed to bloom on into late summer. It is not really hardy here so it barely made it through the winter and just got big enough to put out half a dozen blooms before the snow came. Down there in balmy Chicago it might do just fine.
Leo

Leo, Thank you for the info. I'll check it out.

The hardy Geranium 'Rozanne' stays low; it sprawls, and blooms just about non-stop, lovely. It's a relatively new cultivar, but worth seeking out. Other hardy Geraniums that stay low -- G.cantabrigiense, G.macrorrhizum (great in dry shade, aromatic foliage, evergreen in z6 NJ), G.dalmaticum, and ofcourse G.sanguineum, with red fall foliage.